This invention relates to an electrode assembly. The electrode assembly comprises a substantially planar conductor foil which provides a sealed electric connection through a sealed portion of a lamp envelope. The invention further relates to a method for stiffening a conductor foil of an electrode assembly of a sealed lamp envelope.
A wide variety of lamps with sealed, typically glass or quartz envelopes are known in the art. The envelope contains the electric fittings necessary for the specific physical process which generates the light. A typical example is an incandescent lamp where the sealed envelope contains a glow filament which is heated by electric current flowing through the filament. The current is lead into the envelope by lead wires. Beside the electric fitting, the envelope often also contains a special gas, for example halogen gas such as argon or xenon. A common problem for such lamps is the gas-tight sealing of the glass or quartz around the lead wires, and the exact positioning of the electrode assembly within the sealed envelope. In production, the envelope of the lamp is made from a glass or quartz tube. The lead wires are introduced into the envelope through an open end of the tube, and the end of the tube is pinch sealed, while the material of the tube at the end portion is heated above softening temperature. The sealing of the tube end is done by two pinch jaws which press the softened tube end flat, enclosing the lead-in wires.
It is known that good sealing around the lead-in wires by themselves is practically not achievable. Therefore, it is also known to employ a thin conductor foil in the sealed portion of the envelope. These conductor foils are acting as the sealed lead-through conductors, and the outer and inner lead wires are connected to the conductor foils. The material of the foil is mostly molybdenum. Due to the low thickness of the foil, the softened glass can completely flow around the foil during the pinching, and good, long-term sealing of the envelope is achieved.
However, this sealing technique also poses certain problems for the exact positioning of the electrode assembly. Ideally, the foil should be infinitely thin in order to avoid the stresses caused by the differences in the thermal expansion coefficients of the foil and the glass. In practice, the foils are made very thin, in the order of 20-30 microns. During manufacturing of the lamp, the foils must mechanically support and hold stable the inner lead wires and the attached components, e.g. a glass bead connecting the inner lead wires and the filament welded to the inner lead wires. The low thickness of the foils results in a very easy bending the foil, for example when the electrode assembly is transported on a conveyor line. The chances of an undesired bending of the foil are particularly high during the pinching operation. Such bending results in an offset of the inner lead wires relative to the desired position. Excessive offset of the lead wires or other parts of the electrode assembly may cause early failure of the lamp, or a decrease of the yield, if the offset can not be compensated by a re-positioning of the complete envelope relative to the lamp housing.
In a known method, a protrusion is formed on the pinching jaw that supports from the side the inner lead wire within the indented glass. This solution very much depends on the accurate setting of the pinching jaw, and the natural wear of the jaws must be continuously readjusted. Further, as a result of the protrusion pressing into the glass, the wall thickness of the glass decreases at the indented glass segment which must bear the internal pressure. Also, indentations and protrusions are formed in the glass wall as well. These indentations and protrusions are potential stress sources which may cause cracks in the glass. This may even lead to an explosion of the lamp.
An incandescent lamp with a sealing conductor foil, also termed as a conductor ribbon is disclosed in U.S. Pat. No. 4,295,185. The lamp has a sealed envelope surrounding an electrode assembly. The electrode assembly has outer and inner lead wires which are connected to each other with the conductor ribbon. Tubes are attached to the inner lead wires for receiving the ends of a filament. In order to improve the positioning accuracy of the inner lead wires, and thereby that of the filament, the inner lead wires are connected to each other with an insulating body. However, this means that the conductor ribbons must bear an even greater mass during the assembly of the lamp. No provisions are made to improve the mechanical strength of the conductor ribbons.
Therefore, there is a need for an electrode assembly having conductor foils with an improved mechanical strength, but without impairing the sealing properties of the lead-through conductor foil. Also, there is a need for an electrode assembly for a sealed lamp envelope which would not require complicated manufacturing facilities, but would need only a few additional production steps at most, which could be integrated into the existing production lines in a simple manner.
In an embodiment of the present invention, there is provided an electrode assembly for a sealed lamp envelope. The electrode assembly comprises an outer lead wire for providing an external electric connection terminating outside of the envelope. An inner lead wire of the electrode assembly provides mechanical support and electric connection to an electrode in the envelope. A conductor foil connects the outer lead wire and the inner lead wire. The conductor foil provides a sealed electric connection through a sealed portion of the envelope. In order to improve the mechanical stiffness of the conductor foil, it comprises a curvature. The curvature is formed by the foil in a plane substantially perpendicular to a line connecting the outer lead wire and the inner lead wire. It is most expedient when the curvature is constituted by an edge of a fold. This edge is formed substantially parallel to a common central axis of the outer and inner lead wires.
According to another embodiment of the invention, there is provided a method for stiffening a conductor foil of an electrode assembly of a sealed lamp envelope, where the assembly comprises an outer lead wire for providing an external electric connection terminating outside of the envelope, and an inner lead wire for providing mechanical support and electric connection to electrodes in the envelope, and a substantially planar conductor foil connects the outer lead wire and the inner lead wire. The method comprises the step of providing a curvature in the conductor foil in a plane substantially perpendicular to a line connecting the outer lead wire and the inner lead wire.
The electrode assembly has a higher mechanical stiffness compared with assemblies having flat conductor foils. The suggested electrode assembly improves the positioning accuracy of the electrodes, and lamps with such electrode assemblies have lower fault rates. The improved mechanical stiffness of the foil requires only one additional manufacturing step, so that lamps with such electrode assemblies may be manufactured at practically same cost.